The following discussion is not an admission that anything discussed below is common general knowledge or citable as prior art.
Some water treatment processes use multiple membrane treatment steps in series. In particular, microfiltration (MF) or ultrafiltration (UF) membranes may be used to pre-treat water prior to a nanofiltration (NF) or reverse osmosis (RO) step. Such combined processes are used for example in sea water desalination, wastewater recovery and in some industrial water treatment plants.
For example, a presentation entitled “Desalination Technology Overview” presented by James C. Lozier at the April 2011 Water Resources Research Center Conference in Yuma, Ariz., USA, describes a seawater RO plant. In this plant, there are three trains of UF or MF immersed membranes each having a suction pump delivering permeate to a break tank. From the break tank, water is pumped through a set of parallel RO trains. The RO permeate is stored in tanks and then transferred to a distribution system.